1. Field of the Invention
The invention relates to a pipetting system using a step-and-repeat pipette and a syringe releasably connected thereto.
2. Description of the Prior Art
Pipetting systems of the above-mentioned type serve for proportioning certain liquid volumes (e.g. reagents) and are especially employed at a laboratory. The step-and-repeat pipette is used to hold and actuate the syringe in order to draw liquid into it and dispense the liquid therefrom. The syringe may be replaced with a clean syringe after use and may be disposed of. Suction into the syringe is effected, as a rule, by a single step whereas delivery of certain proportioning volumes may be done by several steps. Pipetting systems using adjustable proportioning volumes have been known before.
Thus, DE 29 26 691 C2 discloses a step-and-repeat pipette which has a rack-actuated pawl means for a step-by-step forward motion of a piston by means of a reciprocating driving lever. The rack further has provided on it an adjustably slidable cover to limit the effective pawl engagement to advance the piston. The cover is adapted to be displaced by a regulating member disposed in the pipette casing, which may be designed with a rotary knob. In this: pipetting system, the proportioning volume can only be adjusted by steps the fineness of which is limited by the pitch of the rack.
Pipetting systems having hydraulic actuators in the widest sense are known from the following publications: DE 40 11 548 A1, DE 42 14 430 A1, DE 25 58 672 A1, U.S. Pat. No. 5,456,885, JP 63-169 565 A, JP 4-161 856 A, Measurement Techniques, Vol. 1987, issue 1, pp. 51-53.
Therefore, it is the object of the invention to create a pipetting system of the aforementioned type which makes it possible to continuously change the proportioning volume.
The inventive pipetting system has a step-and-repeat pipette and a syringe releaseably connected thereto. The step-and-repeat pipette retains the syringe and has means that provides for disfigurement of the syringe piston back away from the syringe aperture and for pushing the syringe piston forward towards the syringe aperture dependent on the liquid volumes to be ejected from the syringe. The providing means includes a working piston connected to the syringe piston and an actuating piston. The working and actuating pistons are sealingly introduced in a working chamber, with the actuating piston having an actuating end which can be actuated from outside. The working piston may be connected directly or indirectly to the syringe piston and the actuating end may be actuated directly or indirectly from outside. A storage chamber is arranged adjacent to the working chamber and has a variable stock volume. The volume may be variable because of an equalizing piston which is sealingly and slidably; introduced in the storage chamber, and/or because of a flexible wall of the storage chamber. In addition, the connection between the working chamber and storage chamber has a valve means which, while the working piston is pushed into the working chamber, especially while the syringe piston is pushed back, allows a hydraulic fluid to pass from the working chamber to the storage chamber. As a result, the liquid volume which is displaced by the working piston while it is pushed it is pushed in is caught by the variable stock . volume of the storage chamber. This makes it possible to charge the syringe with liquid and/or to adjust the working piston in an initial position for the delivery of liquid. In addition, the valve means separates the working chamber from the storage chamber while the actuating piston is pushed into the former in order that the hydraulic liquid may force the working piston out of the working chamber by one step with a view to pushing the syringe piston forward. Separation of the working chamber from the storage chamber will prevent the liquid volume displaced by the actuating piston from being received by the variable stock volume without forcing out the working piston. However, while the actuating piston is pushed back from the working chamber the valve means allows hydraulic liquid to pass from the storage chamber over to the working chamber for refilling. This will replace a volume of hydraulic fluid, which corresponds to the motion of the working piston out of the working chamber so that the working piston is forced out by another step while the actuating piston is actuated next time.
The liquid volume dispensed from the syringe will be dependent upon the distance by which the actuating piston is pushed into the working chamber. Thus, the proportioning volume is infinitely variable. Preferably, the actuating piston is adapted to be moved between a first stop limiting the push-back motion and a second stop limiting the pushing-in motion, which creates reference points for the proportioning volume. When the actuating piston is moved from one stop to the other the proportioning volume is precisely defined. Also preferably, the spacing between the first and second stops may be adjusted by means of an adjustment means. This also helps adjust the volume to be proportioned, which is achieved by moving the actuating piston from one stop to the other. The adjustment means may have a micrometer means and may be coupled to an indicator means for the proportioning volume. Preferably, the actuating piston is biased by a spring in the push-back direction so that it will independently reset itself, after undergoing actuation, to an initial position. for another proportioning step.
The valve means may have a check valve, which opens in the direction of flow from the storage chamber to the working chamber and closes in a reverse direction. The check valve automatically causes a separation of the storage chamber and working chamber while the actuating piston is pushed into the working chamber and automatically eliminates such separation while the actuating piston is drawn out. Moreover, the check valve may be adapted to be actuated from outside for the passage of hydraulic liquid while the working piston is pushed in. In lieu of an active check valve, there may be an extra valve adapted to be actuated from outside between the working chamber and storage chamber.
A springback of the syringe piston sealing lip following a proportioning step may result in the syringe sucking back a small liquid volume. To avoid faulty proportioning, a first working chamber and a second working chamber may exist, the working piston being associated with the first working chamber and being connected to the storage chamber via a valve of the valve means, which may be opened while the working piston is pushed into the first working chamber. Further, the second working chamber may have associated with it the actuating piston and may be connected to the storage chamber via a valve of the valve means, which separates it from the second working chamber while the actuating piston is pushed in and connects it to the second working chamber while the actuating piston is pushed back. The further valve may be a check valve. The working chambers are interconnected by a further valve means which closes upon completion of the forward motion of the actuating piston. The further valve means may be a further check valve which opens in the direction of flow from the second working chamber to the first working chamber and closes in a reverse direction. When the actuating piston is then relieved and withdrawn the working piston will not be relieved, however, but will be kept under a tension by the closing procedure of the second valve means in the first working chamber, as will the syringe piston. On the other hand, the second working chamber will fill while the actuating piston is withdrawn with the hydraulic liquid volume required. This volume is forced into the first working chamber during the next actuation by the further valve means so that the syringe will at any time dispense the proper proportioning volume.
In addition, there may be a means for avoiding a residual lift, which prevents another proportioning step when the residual liquid volume existing in the syringe is smaller than the adjusted proportioning volume. To this end, the means for avoiding a residual lift may have a means for sensing the position of the working piston while the actuating piston is pushed back, and a means for comparing the residual lift determined to the volume to be proportioned as well as a means for preventing the working piston from further being pushed out when the residual lift is insufficient. Sensing the position of the working piston only during each backward motion of the actuating piston will achieve that the last proportioning procedure possible may still be effected completely. Blocking the further motion of the working piston will achieve that the succeeding proportioning action can no more be started. Preferably, in addition, the blockage of a further motion of the working piston is coupled by the means for avoiding a residual lift to the setting procedure of the adjustment means so that the respective proportioning volume will then be taken into account. Also preferably, the actuating piston will not be blocked when the operation to avoid a residual lift becomes effective, but its actuation is possible with no displacement of the working piston.
Non-defined proportioning volumes may be dispensed when the actuating piston is relieved in the meantime. Therefore, another aspect of the invention provides that there is a means ensuring that the working piston is pushed out of the working chamber, which prevents the actuating piston from moving back and/or the valve means from opening before the completion of the proportioning step. Here, the means ensuring that the actuating piston reaches the second stop is adapted to effect sensing, which is an indication that the proportioning step is fully completed.